U.S. patent number 6,900,473 [Application Number 10/178,714] was granted by the patent office on 2005-05-31 for surface-emitting semiconductor light device.
This patent grant is currently assigned to Kabushiki Kaisha Toshiba. Invention is credited to Hideki Sekiguchi, Koichi Takahashi, Kazuhiro Takimoto, Atsuko Yamashita, Shunji Yoshitake.
United States Patent |
6,900,473 |
Yoshitake , et al. |
May 31, 2005 |
Surface-emitting semiconductor light device
Abstract
A semiconductor light emitting device is disclosed in which a
semiconductor multilayer structure including a light emitting layer
is formed on a substrate and light is output from the opposite
surface of the semiconductor multilayer structure from the
substrate. The light output surface is formed with a large number
of protrusions in the form of cones or pyramids. To increase the
light output efficiency, the angle between the side of each
protrusion and the light output surface is set to between 30 and 70
degrees.
Inventors: |
Yoshitake; Shunji (Kawasaki,
JP), Sekiguchi; Hideki (Yokohama, JP),
Yamashita; Atsuko (Yokosuka, JP), Takimoto;
Kazuhiro (Toyoura-gun, JP), Takahashi; Koichi
(Yokohama, JP) |
Assignee: |
Kabushiki Kaisha Toshiba
(Tokyo, JP)
|
Family
ID: |
26617516 |
Appl.
No.: |
10/178,714 |
Filed: |
June 25, 2002 |
Foreign Application Priority Data
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Jun 25, 2001 [JP] |
|
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2001-191724 |
Sep 27, 2001 [JP] |
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2001-297042 |
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Current U.S.
Class: |
257/95; 257/100;
257/103; 257/81; 257/98; 257/99; 257/E33.074 |
Current CPC
Class: |
H01L
33/22 (20130101); H01L 33/44 (20130101); H01L
33/20 (20130101); H01L 33/38 (20130101) |
Current International
Class: |
H01L
33/00 (20060101); H01L 033/00 () |
Field of
Search: |
;257/81,95,98,99,100,103,46 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0 404 565 |
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Dec 1990 |
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EP |
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63-283174 |
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Nov 1988 |
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JP |
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7-183575 |
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Jul 1995 |
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JP |
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11-204840 |
|
Jul 1999 |
|
JP |
|
WO 01/18883 |
|
Sep 2000 |
|
WO |
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WO 01/24280 |
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Sep 2000 |
|
WO |
|
Other References
Reiner Windisch et al., "40% Efficient Thin-Film Surface-Textured
Light-Emitting Diodes by Optimization of Natural Lithography", IEEE
Transactions on Electron Devices, vol. 47, No. 7, Jul. 2000, pp.
1492-1498. .
European Search Report..
|
Primary Examiner: Tran; Minhloan
Assistant Examiner: Tran; Tan
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt, P.C.
Claims
What is claimed is:
1. A surface-emitting semiconductor light emitting device
comprising: a substrate having a major surface; and a semiconductor
multilayer structure formed on the major surface of the substrate
and including a light emitting layer, visible light being output
from the opposite surface of the semiconductor multilayer structure
and the light output surface having been subjected to a roughening
process so that a large number of protrusions and recesses is
formed thereon, said protrusions and recesses defining peaks and
valleys and a height between the maximum peak and the lowest valley
being between 50 nm and a wavelength of visible light emitted by
the light emitting layer, wherein the protrusions and recesses are
formed periodically with a period set to 0.5 .lambda. or less,
where .lambda. is the wavelength of the emitted visible light.
2. The device according to claim 1, wherein the semiconductor
multilayer structure has a double heterostructure in which an
active layer is sandwiched between cladding layers, a transparent
electrode is formed on the opposite cladding layer of the double
heterostructure from the substrate, and a surface of the cladding
layer immediately under the transparent electrode has been
subjected to the roughening process.
3. The device according to claim 2, wherein the active layer is
made of InGaAlP and the cladding layers are each made of InAlP.
4. The device according to claim 1, wherein the semiconductor
multilayer structure has a double heterostructure in which, an
active layer is sandwiched between cladding layers, a current
diffusing layer is formed on the opposite cladding layer of the
double heterostructure from the substrate, and the surface of the
current diffusing layer has been subjected to the roughening
process.
5. The device according to claim 1, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is within a range in which a graded index effect is
ensured.
6. The device according to claim 1, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is set to 0.2 .mu.m or less.
7. A surface-emitting semiconductor light emitting device
comprising: a substrate having a major surface; a semiconductor
multilayer structure formed on the major surface of the substrate
and including a light emitting layer, visible light being output
from the opposite surface of the multilayer structure from the
substrate; and an antireflection film formed on the light output
surface of the semiconductor multilayer structure and having its
surface roughened so that a large number of protrusions and
recesses is formed thereon, said protrusions and recesses defining
peaks and valleys and a height between the maximum peak and the
lowest valley being between 50 nm and a wavelength of visible light
emitted by the light emitting layer, wherein the protrusions and
recesses are formed periodically with a period set to 0.5 .lambda.
or less, where .lambda. is the wavelength of the emitted visible
light.
8. The device according to claim 7, wherein a refractive index of
the antireflection film is set higher than that of a transparent
resin which is applied to the light output surface of the
semiconductor multilayer structure but lower than that of a top
layer of the semiconductor multilayer structure.
9. The device according to claim 7, wherein the semiconductor
multilayer structure has a double heterostructure in which an
active layer is sandwiched between cladding layers and a current
diffusing layer is formed on the opposite cladding layer of the
double heterostructure from the substrate.
10. The device according to claim 9, wherein the active layer is
made of InGaAlP and the cladding layers are each made of InAlP.
11. The device according to claim 7, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is within a range in which a graded index effect is
ensured.
12. The device according to claim 7, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is set to 0.2 .mu.m or less.
13. A surface-emitting semiconductor light emitting device
comprising: a substrate having a major surface; a semiconductor
multilayer structure formed on the major surface of the substrate
and including a light emitting layer, visible light being output
from the opposite surface of the multilayer structure from the
substrate; a first electrode formed in selected areas of the light
output surface of the semiconductor multilayer structure; an
antireflection film formed on the light output surface of the
semiconductor multilayer structure except the areas of the first
electrode and has its surface roughened so that a large number of
protrusions and recesses is formed thereon; a second electrode
formed over the entire rear surface of the substrate; and said
protrusions and recesses defining peaks and valleys, and a height
between the maximum peak and the lowest valley being between 50 nm
and a wavelength of visible light emitted by the light emitting
layer, wherein the protrusions and recesses are formed periodically
with a period set to 0.5 .lambda. or less, where .lambda. is the
wavelength of the emitted visible light.
14. The device according to claim 13, wherein a refractive index of
the antireflection film is set higher than that of a transparent
resin which is applied to the light output surface of the
semiconductor multilayer structure but lower than that of a top
layer of the semiconductor multilayer structure.
15. The device according to claim, 13, wherein the semiconductor
multilayer structure has a double heterostructure in which an
active layer is sandwiched between cladding layers and a current
diffusing layer is formed on the opposite cladding layer of the
double heterostructure from the substrate.
16. The device according to claim 15, wherein the active layer is
made of InGaAlP and the cladding layers are each made of InAlP.
17. The device according to claim 13, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is within a range in which a graded index effect is
ensured.
18. The device according to claim 13, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is set to 0.2 .mu.m or less.
19. A surface-emitting semiconductor light emitting device
comprising: a substrate made of a compound semiconductor of a first
conductivity type; a double heterostructure formed on the substrate
and comprising a cladding layer of the first conductivity type, an
active layer, and a cladding layer of a second conductivity type; a
current diffusing layer of the second conductivity type formed on
the cladding layer of the second conductivity type.of the double
heterostructure; a contact layer of the second conductivity type
formed on the current diffusing layer; an upper electrode
selectively formed on the contact layer; a lower electrode formed
on the rear surface of the substrate; and an antireflection film
formed on the contact layer except its portions where the upper
electrode is formed, the antireflection film having its surface
roughened so that a large number of neighboring protrusions and
recesses is formed thereon, said protrusions and recesses defining
peaks and valleys, and a height between the maximum peak and the
lowest valley being between 50 nm and a wavelength of visible light
emitted by the light emitting layer, and the protrusions and
recesses are formed periodically with a period set to 0.5 .lambda.
or less, where .lambda. is the wavelength of the emitted
visible.
20. The device according to claim 19, wherein the protrusions and
recesses are formed at a regular interval and a pitch of the
protrusions is within a range in which a graded index effect is
ensured.
21. The device according to claim the protrusions and recesses are
formed at a regular interval and a pitch of the protrusions is set
to 0.2 .mu.m or less.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority
from the prior Japanese Patent Applications No. 2001-191724, filed
Jun. 25, 2001; and No. 2001-297042, filed Sep. 27, 2001, the entire
contents of both of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor light emitting
diode, such as a light emitting diode (LED) or a laser diode (LD).
More specifically, the present invention relates to a semiconductor
light emitting device having its light output surface made
rough.
2. Description of the Related Art
Conventionally, a high intensity LED has been fabricated by forming
a double-heterostructure light emitting region on a semiconductor
substrate and then forming a current diffusing layer on the light
emitting region. For this reason, packaging the high intensity LED
with a resin results in a structure in which the top of the current
diffusing layer is covered with the passivating transparent
resin.
With such a structure, the critical angle associated with the
current diffusing layer (the refractive index is in the range of
3.1 to 3.5) and the vitreous resin (the refractive index is of the
order of 1.5) is in the range of 25 to 29 degrees. Of light that
travels from the light emitting region toward the vitreous resin,
the light that strikes the layer--resin interface at angles larger
than the critical angle will suffer total internal reflections.
This will significantly reduce the probability of light produced
within the LED being emitted to the outside. At present, the
probability (light output efficiency) is of the order of 20%.
There is a method of improving the light output efficiency by
forming a film of high refractive index on the current diffusing
layer to thereby increase the critical angle. However, even with
this method, an increase in the efficiency is low, of the order of
20%.
Thus, the conventional LEDs that are packaged with transparent
resin material suffer from the problem that the light output
efficiency is low. This is because, at the interface between the
transparent resin and the top layer of semiconductor multi-layer
structure including a light emitting layer, most of the light that
strikes the interface at angles suffers total internal reflections.
This problem is common to surface-emitting LDs.
BRIEF SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided
a surface-emitting semiconductor light emitting device comprising;
a substrate having a major surface; a semiconductor multilayer
structure formed on the major surface of the substrate and
including a light emitting layer, emitted light being output from
the opposite surface of the multilayer structure from the
substrate; and a plurality of protrusions formed on the light
output surface of the semiconductor multilayer structure, the angle
between the base and side of each protrusion being set to between
30 and 70 degrees.
According to another aspect of the present invention, there is
provided a surface-emitting semiconductor light emitting device
comprising: a substrate having a major surface; and a semiconductor
multilayer structure formed on the major surface of the substrate
and including a light emitting layer, light being output from the
opposite surface of the semiconductor multilayer structure and the
light output surface having been subjected to a roughening process
so that a large number of protrusions and recesses is formed
thereon, the distance between the peak and valley of each
protrusion and recess being set to between 50 nm and the wavelength
of light emitted by the light emitting layer.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING
FIGS. 1A, 1B and 1C are cross-sectional views, in the order of
steps of manufacture, of a green LED according to a first
embodiment of the present invention;
FIG. 2 is an enlarged cross-sectional view of protrusions formed on
the light output surface of the LED of FIG. 1;
FIG. 3 is a plan view of an electrode pattern of the LED of FIG.
1;
FIG. 4 is a plot of the angle between the side of the protrusion
and the substrate surface versus the light output efficiency of the
LED of FIG. 1;
FIG. 5 is a cross-sectional view of a green LED according to a
second embodiment of the present invention;
FIGS. 6A and 6B are cross-sectional views, in the order of steps of
manufacture, of a green LED according to a third embodiment of the
present invention;
FIG. 7 is an enlarged cross-sectional view of the neighborhood of
the light output surface in the third embodiment;
FIG. 8 is a cross-sectional view of a surface-emitting LD according
to a third embodiment of the present invention;
FIGS. 9A, 9B and 9C are cross-sectional views, in the order of
steps of manufacture, of a green LED according to a fifth
embodiment of the present invention;
FIG. 10 shows a plot of the light output efficiency versus the
height of protrusions in the LED of FIG. 5;
FIG. 11 shows a plot of the light output efficiency versus the
height of protrusions comparable in size with the emitted
wavelength;
FIG. 12 shows a plot of the light output efficiency versus the
refractive index when the surface of the antireflection film is
roughened;
FIG. 13 shows a plot of the light output efficiency versus the
refractive index when the surface of the antireflection film is
made smooth;
FIGS. 14A to 14E are sectional views illustrating various surface
configurations of the antireflection film which may be used in the
invention;
FIG. 15 is a cross-sectional view of a green LED according to a
sixth embodiment of the present invention; and
FIG. 16 is a cross-sectional view of a surface-emitting LD
according to a seventh embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The preferred embodiments of the present invention will be
described with reference to the accompanying drawings.
FIGS. 1A, 1B and 1C are cross-sectional views, in the order of
steps of manufacture, of a green LED according to a first
embodiment of the present invention.
First, as shown in FIG. 1A, onto an n-type GaAs substrate 10 of 250
.mu.m in thickness, an n-type GaAs buffer layer 11 of 0.5 .mu.m in
thickness is grown by means of metal-organic CVD (MOCVD) using
AsH.sub.3 as a group V element source gas. After that, by means of
MOCVD using PH.sub.3 as a group V element source gas and under
conditions of a PH.sub.3 partial pressure of 200 Pa and a total
pressure of 5.times.10.sup.3 Pa, an n-type In.sub.0.5 Al.sub.0.5 P
cladding layer 12 of 0.6 .mu.m in thickness and an undoped
In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer 13 of
1.0 .mu.m in thickness are grown in sequence.
Subsequently, a p-type In.sub.0.5 Al.sub.0.5 P cladding layer 14 of
1.0 .mu.m in thickness is grown by means of MOCVD with the PH.sub.3
partial pressure reduced to 10 Pa and the total pressure kept at
5.times.10.sup.3 Pa. After that, a p-type GaAs contact layer 16 of
0.1 .mu.m in thickness is grown by means of MOCVD using AsH.sub.3
as a group V element source gas. Each of the epitaxial layers from
the buffer layer 11 to the contact layer 16 is grown in succession
in the same chamber.
As described above, in growing the p-type InAlP cladding layer 14,
when the PH.sub.3 partial pressure in the MOCVD process is reduced
to a sufficiently low pressure (not higher than 20 Pa), the surface
of the epitaxial layer becomes roughened. To be specific, conical
protrusions 20 are produced on the surface of the InAlP cladding
layer 14 as shown in FIG. 2. The angle of each protrusion with
respect to the substrate surface, i.e., the angle .alpha. made by
the base and the side of each protrusion, becomes larger than 30
degrees.
Here, if, when the InAlP cladding layer 14 is being grown, the
PH.sub.3 partial pressure is in excess of 20 Pa, the surface of the
cladding layer would become less roughened, increasing the
possibility that each protrusion may fail to attain more than 30
degrees in the angle .alpha. made by its base and side. If, on the
other hand, the PH.sub.3 partial pressure is lower than 1 Pa, then
the surface of the cladding layer 14 would become too much
roughened and moreover its crystallinity would also become
degraded. Therefore, the PH.sub.3 partial pressure at the growth of
the InAlP cladding layer 14 should preferably be in the range of 1
to 20 Pa.
Next, as shown in FIG. 1B, an ITO (Indium Thin Oxide) film 17
serving as a transparent electrode is formed on a selected portion
of the GaAs contact layer 16 through sputtering techniques.
Subsequently, a p-side electrode (Zn-containing Au) 23 is formed on
the ITO film 17. More specifically, a current block layer 21 and a
GaAs layer 22 are grown on the ITO film 17 and then selectively
etched so that they are left in the central area of the chip.
Subsequent to this process, the AuZn electrode 23 is formed over
the entire surface and then patterned so that it is left on the
GaAs film 22 and selected portions of the ITO film 17.
FIG. 3 is a plan view illustrating a pattern of the p-side
electrode 23. This electrode pattern is comprised of a circular pad
23a provided in the central area of the device so that a wire may
be bonded, peripheral portions 23 provided along the edges of the
device, and contact portions 23c that connect the peripheral
portions 23b to the central pad 23a.
Next, as shown in FIG. 1C, the GaAs substrate 10 has its rear side
polished to a thickness as small as 100 .mu.m and then formed
underneath with an n-side electrode (Ge-containing Au) 25. After
that, the resulting structure is subjected to a heat treatment in
an Ar atmosphere at 450.degree. C. for 15 minutes. Subsequently,
the substrate 10, formed with the layers 11, 12, 13, 14, 16, 17, 21
and 22 and the electrodes 23 and 25, is scribed to obtain chips.
Each individual chip is then housed in a resin package so that its
light output surface is covered with a transparent resin not
shown.
A single chip structure is illustrated in FIG. 1; however, in
practice a plurality of chip structures as shown in FIG. 1 is
formed on the same substrate 10 in order to form a plurality of
chips at the same time. By scribing the substrate 10 at the final
stage, it is separated into chips.
According to the present embodiment, as described above, the
conical protrusions 20 can be formed on the surface of the cladding
layer 14 by setting the PH.sub.3 partial pressure lower than usual
when the p-type InAlP cladding layer 14 is grown. The formation of
the protrusions 20 allows the probability of incident light
suffering total internal reflections at the interface between the
topmost layer of the multi-layer structure including the light
emitting layer and the transparent resin to be reduced. In
particular, by setting the InAlP cladding layer growth time
PH.sub.3 partial pressure to between 1 and 20 Pa, the angle a made
by the base and the side of each protrusion can be set to 30
degrees or more.
Here, a relationship between the incidence-on-resin probability
(light output efficiency) and the angle of the protrusions 20 with
respect to the substrate surface is shown in FIG. 4. In this
figure, the angle is shown on the horizontal axis and the light
output efficiency is shown on the vertical axis. The light output
efficiency when the surface is free of protrusions and hence flat
is taken to be unity. An improvement of more than ten percent was
recognized when the angle .alpha. was 30 degrees or more.
Conversely, when the angle .alpha. was too large, a reduction in
the efficiency was recognized. With angles in excess of 70 degrees,
the efficiency fell below ten percent. Thus, the angle .alpha.
should preferably range from 30 to 70 degrees.
The adoption of the protrusion structure as in this embodiment
allows the light output efficiency to be increased by a factor of
1.15 in comparison with the conventional device without
protrusions. That the light output efficiency can be increased
without changing the basic device structure is extremely
advantageous to LEDs.
Even though the angle .alpha. should be set to 30 degrees or more,
all the protrusions need not necessarily meet this requirement.
Most of the protrusions (for example, more than 90 percent) have
only to meet the requirement. Even if each protrusion is formed so
as to have an angle .alpha. in the range of 30 to 70 degrees, some
of the resulting protrusions may have an angle of less than 30
degrees and some of them may have an angle of more than 70 degrees.
There will arise no problem if the percentage of protrusions that
have angles outside the range of 30 to 70 degrees is sufficiently
low.
Thus, according to the present embodiment, the light output
efficiency can be significantly improved by setting the angle
.alpha. made by the base and the side of each protrusion to between
30 and 70 degrees, not by simply making the light output surface
rough.
When the pitch or period of the protrusions 20 formed on the light
output side is made extremely small, the effect of increasing the
light output efficiency is reduced. According to our experiments,
satisfactory effects were confirmed when the pitch of the
protrusions was 0.5 .mu.m or more. The current block layer 21 and
the GaAs layer 22 on the transparent electrode 17 are not
necessarily required. Even when the metal electrode 23 was directly
formed on the transparent electrode 17, we confirmed similar
effects.
[Second Embodiment]
Referring now to FIG. 5, there is illustrated, in sectional view,
the structure of a green LED according to a second embodiment of
the present invention.
In the second embodiment, each of grown layers is opposite in
conductivity type to a corresponding one of the grown layers in the
first embodiment and the basic structure and the method of
manufacture of the LED remain unchanged from those of the first
embodiment.
Onto a p-type GaAs substrate 30 are sequentially grown by means of
MOCVD a p-type GaAs buffer layer 31 of 0.5 .mu.m in thickness, a
p-type In.sub.0.5 Al.sub.0.5 P cladding layer 32 of 0.6 .mu.m in
thickness, an undoped InGaAlP active layer 33 of 1.0 .mu.m in
thickness, an n-type In.sub.0.5 Al.sub.0.5 P cladding layer 34 of
1.0 .mu.m in thickness, and an n-type GaAs contact layer 36 of 0.1
.mu.m in thickness. A transparent ITO film 37 is then formed on the
contact layer 36 by means of sputtering techniques.
As in the first embodiment, in growing the n-type InAlP cladding
layer 34, the PH.sub.3 partial pressure in the MOCVD process is
reduced to a sufficiently low pressure (20 Pa or below). Thereby,
conical protrusions are produced on the surface of the InAlP
cladding layer 34 as in the first embodiment. The angle .alpha. of
each protrusion with respect to the substrate surface becomes 30
degrees or more.
A current block layer 41 and a GaAs layer 42 are formed on a
selected portion of the ITO film 37 and an n-side electrode 43
consisting of AuGe is formed on selected portions of the GaAs layer
42 and the ITO film 37. The GaAs substrate 30 is formed underneath
with a p-type electrode 45 made of ZnAu.
With such a structure as described above, the conical protrusions
formed on the surface of the n-type InAlP cladding layer 34 allows
the probability of incidence of light on the transparent resin for
packaging to be increased as in the first embodiment.
[Third Embodiment]
FIGS. 6A and 6B are sectional views, in the order of steps of
manufacture, of a green LED according to a third embodiment of the
present invention.
First, as shown in FIG. 6A, onto an n-type GaAs substrate 50 of 250
.mu.m in thickness are sequentially grown by means of MOCVD an
n-type In.sub.0.5 Al.sub.0.5 P cladding layer 12 of 0.6 .mu.m in
thickness, an undoped In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5
P active layer 53 of 1.0 .mu.m in thickness, a p-type In.sub.0.5
Al.sub.0.5 P cladding layer 54 of 1.0 .mu.m in thickness, an n-type
InGaP current diffusing layer 55 of 3.0 .mu.m in thickness, and a
p-type GaAs contact layer 56 of 0.1 .mu.m in thickness. For
epitaxial growth of these layers, the MOCVD techniques are used as
in the first embodiment.
Next, an annealing step is performed at a temperature equal to or
higher than the epitaxial temperature (not lower than 600.degree.
C.) in order to change the epitaxial surface topography. Thereby,
the surface of the current diffusing layer 55 becomes roughened to
form protrusions. Afterward, a p-side electrode 63 is formed on the
current diffusing layer 55 and an n-side electrode 65 is formed on
the back of the substrate 50. Subsequently, the exposed portion of
the p-GaAs layer 56 is removed. Thus, the structure shown in FIG.
6B is completed.
Here, the surface roughening by annealing will be described in more
detail. As the gases used in the annealing step, an inert gas, such
as hydrogen, and a group V element gas (for example, AsH.sub.3)
different from group V elements (for example, p) constituting the
epitaxial films (III-V compound materials, for example, InGaAlP)
are introduced. The group V element (P) in the epitaxial surface
layer is reevaporated. Further, as the next step, an epitaxial step
is performed on the roughened surface (the type of film: a
transparent film of, say, GaP).
Thus, P is extracted from the surface of the InGaP current
diffusing layer 55, so that the surface becomes roughened as shown
in FIG. 7. A transparent GaP layer 57 is then grown on the rough
surface of the InGaP layer 56. The desired surface topography for
increasing the light output efficiency is one in which a large
number of convex conical protrusions are formed over the surface,
which is obtained from a conventional epitaxial surface in a state
of mirror surface (Rmax=5 nm). The angle of each conical protrusion
with respect to the base is larger than 30 degrees.
With such a structure as well, the conical protrusions formed on
the surface of the current diffusing layer 55 allows the
probability of incidence of light on the transparent resin for
packaging to be increased as in the first embodiment.
The p-type GaAs contact layer 56 need not necessarily be removed;
however, if it absorbs light of the emitted wavelength, it should
preferably be removed.
[Fourth Embodiment]
In FIG. 8, there is illustrated, in sectional view, the structure
of a surface-emitting LD according to a fourth embodiment of the
present invention.
First, on an n-type GaAs substrate 70 of 250 .mu.m in thickness are
sequentially grown an n-type GaAs buffer layer 71 of 0.5 .mu.m in
thickness and a DBR reflecting layer 78 consisting of stacked
n-In.sub.0.5 Al.sub.0.5 P/n-GaAs films.
Subsequently, an n-type In.sub.0.5 Al.sub.0.5 P cladding layer 72
of 0.6 .mu.m in thickness, an undoped MQW active layer 73 of
In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P/In.sub.0.5
Ga.sub.0.5 P, and a p-type In.sub.0.5 Al.sub.0.5 P cladding layer
74 of 0.6 .mu.m in thickness are grown in sequence, thus forming a
double heterostructure. Subsequently, a DBR reflecting layer 79
consisting of stacked p-In.sub.0.5 Al.sub.0.5 P/p-GaAs films, a
p-type In.sub.0.5 Al.sub.0.5 P current diffusing layer 76 of 1.0
.mu.m in thickness and a p-type GaAs contact layer 77 of 0.1 .mu.m
in thickness are grown in sequence.
Each of the epitaxial layers from the buffer layer 71 to the
contact layer 77 is grown in succession in the same chamber through
the use of MOCVD techniques. The type of gas used and the pressure
thereof are selected so that each layer is grown well. In growing
the current diffusing layer 76, as in the first embodiment, the
PH.sub.3 partial pressure is reduced to a sufficiently low value
of, for example, 10 Pa so as to allow the layer surface to become
roughened.
Next, a resist pattern is formed on the contact layer 77 and the
layers through the n-type cladding layer 72 are then etched away
using the resist pattern as a mask to thereby form a laser ridge.
Subsequently, an insulating film 81 is formed except the top of the
ridge and then a p-type electrode (Zn-containing Au) is evaporated.
Using a resist mask, a portion of the p-type electrode which is
located in the central portion of the ridge and the p-GaAs contact
layer 77 underlying that portion of the p-type electrode are
removed, thereby forming an upper electrode 83. Subsequently, the
GaAs substrate 70, after having its rear side polished to a
thickness of 100 .mu.m, is formed underneath with an n-side
electrode (Ge-containing Au) 85. Next, a heat treatment is carried
out in an Ar atmosphere at 450.degree. C. for 15 minutes.
Subsequently, the substrate 70 is scribed to obtain chips. Each
individual chip is then housed in a resin package.
According to the fourth embodiment thus configured, reducing the
PH.sub.3 partial pressure at the growth of the p-type current
diffusing layer 76 allows protrusions (irregularities) to be formed
on the surface of that current diffusing layer and the angle
between the surface of the resulting conical protrusions and the
base to be made larger than 30 degrees. In the fourth embodiment,
as in the first embodiment, the light output efficiency can
therefore be increased. Although the laser diode of the fourth
embodiment is adapted to emit red light, this is not restrictive.
We confirmed the above effects for other laser diodes than red
diodes.
The p-type GaAs contact layer 77 need not necessarily be removed;
however, if it absorbs light of the emitted wavelength, it should
preferably be removed.
[Fifth Embodiment]
FIGS. 9A, 9B and 9C are sectional views, in the order of steps of
manufacture, of a green LED according to a fifth embodiment of the
present invention.
First, as shown in FIG. 9A, onto an n-type GaAs substrate 110 of
250 .mu.m in thickness, an n-type GaAs buffer layer 11 of 0.5 .mu.m
in thickness is grown by means of MOCVD using AsH.sub.3 as a group
V element source gas. After that, by means of MOCVD using PH.sub.3
as aV group element source gas and under conditions of a PH.sub.3
partial pressure of 200 Pa and a total pressure of 5.times.10.sup.3
Pa, an n-type In.sub.0.5 Al.sub.0.5 P cladding layer 112 of 0.6
.mu.m in thickness, an undoped InGaAlP active layer 113 of 1.0
.mu.m in thickness, a p-type In.sub.0.5 Al.sub.0.5 P cladding layer
114 of 1.0 .mu.m in thickness and a p-type InGaP current diffusing
layer 115 of 1.0 .mu.m in thickness are grown in sequence.
Subsequently, a p-type GaAs contact layer 116 of 0.1 .mu.m in
thickness is grown by means of MOCVD using AsH.sub.3 as a group V
element source gas. Each of the epitaxial layers from the buffer
layer 111 to the contact layer 116 is grown in succession in the
same chamber.
Next, as shown in FIG. 9B, an antireflection film 117 is formed,
which features this embodiment. That is, the antireflection film
117 having a refractive index of 2.0 and prepared by adding
TiO.sub.2 to a polyimide resin is formed on the contact layer 116
by spin coating and its surface is then press shaped by a metal
mold having protrusions which are less in size than the wavelength
of emitted light. Thereby, the surface roughness (PV value
(max-min)) of the antireflection film 117 is set to be less than
the wavelength of emitted light. Here, the PV value refers to the
distance (height) between the peak and the valley of each
protrusion.
Next, the antireflection film 117 is formed on top with a resist
mask (not shown) and then removed by RIE in the place where an
upper electrode is to be formed. The resist mask is then removed.
Subsequently, as shown in FIG. 9C, an electrode material
(Zn-containing Au) is evaporated onto the antireflection film 117
and the exposed contact layer 116 and then patterned using a resist
mask (not shown), thus forming the upper electrode (p-side
electrode) 118. The pattern of the p-side electrode 118 remains
unchanged from that shown in FIG. 3.
Next, the GaAs substrate 110 has its rear side polished to a
thickness of 100 .mu.m and then formed underneath with a lower
electrode 119 (Ge-containing Au) serving as the n-side electrode.
After that, the resulting structure is subjected to a heat
treatment in an Ar atmosphere at 450.degree. C. for 15 minutes.
Subsequently, the substrate 110 is scribed to obtain chips. Each
individual chip, after wire bonding, is encapsulated with
epoxy-based resin (n is about 1.5).
Thus, according to the sixth embodiment, by causing the surface of
the antireflection film 117 to become roughened, the light output
efficiency was increased from about 20% (the value of the
conventional device) to about 30%. That is, the light output
efficiency was increased by a factor of 1.15 in comparison with the
conventional device. That the light output efficiency can be
increased by such an amount without changing the basic device
structure is extremely advantageous to LEDs.
FIG. 10 shows the relationship between the PV value and the light
output efficiency. As the PV value increases, the light output
efficiency increases. When the PV value exceeds 50 nm, the light
output efficiency becomes 1.5 or more. When the PV value exceeds
200 nm, the light output efficiency becomes about 2 and remains
almost constant. FIG. 11 shows the relationship between the PV
values corresponding to wavelengths including the wavelength of
emitted light and the light output efficiency. At PV values
corresponding to wavelengths shorter than 640 nm, the emitted
wavelength, a sufficient light output efficiency is obtained.
However, when the PV value increases above the value corresponding
to the emitted wavelength, the light output efficiency decreases
sharply. Therefore, the PV value should preferably be ranged from
200 nm to less than a value corresponding to the emitted
wavelength.
Note that all the protrusions and recesses need not necessarily
meet the requirement that the PV value be ranged from 200 nm to a
value corresponding to the emitted wavelength and most of them (for
example, not less than 90%) have only to meet the requirement. That
is, even if the protrusions and recesses are formed so as to
satisfy the requirement that 200 nm.ltoreq.PV.ltoreq.emitted
wavelength, some of them may be outside the range. If the
percentage of such protrusions and recesses is low enough, no
problem will arise.
FIG. 12 shows the relationship between the light output efficiency
and the refractive index when the surface of the antireflection
film is made rough. This indicates the percentage of light that is
output from a surface of the antireflection film when light falls
on the other surface of that film at an angle of incidence of -90
to +90 degrees. From FIG. 12 it can be seen that, when reference is
made to the light output efficiency at a refractive index of 1.5
(the same as that of the underlying semiconductor layer), the light
output efficiency is increased by about 50% at a refractive index
of 2.0 (this embodiment) and by about 100% at a refractive index of
2.5.
FIG. 13 shows the relationship between the light output efficiency
and the refractive index when the surface of the antireflection
film is smoothed. In this case, the light output efficiency is
increased by 8% at a refractive index of 2.0. Even at a refractive
index of 2.5, an increase in the light output efficiency is as low
as 9%. From this it can bee seen that, in order to increase the
light output efficiency, it is necessary not only to increase the
refractive index of the antireflection film but also to make its
surface rough.
Our experiments confirmed that the light output efficiency was
increased sufficiently by setting the surface roughness (PV value
(max-min)) of the antireflection film to emitted wavelength
.lambda. or less. Further, our experiments confirmed that, as the
surface topology of the antireflection film, the formation of cones
or polygonal pyramids (triangular pyramids, rectangular pyramids,
hexagonal pyramids, etc.) at a pitch of 0.5 .lambda. or less
offered more successful results.
Thus, according to this embodiment, the probability of incident
light suffering total internal reflections at the interface between
the top layer of the semiconductor multilayer structure including a
light emitting layer and the transparent resin can be reduced by
forming an antireflection film whose surface is rough on the light
output side of the semiconductor multilayer structure. Also, it
becomes possible to increase the light output efficiency
significantly by setting the surface roughness of the
antireflection film to the emitted wavelength or less. In addition,
by setting the refractive index of the antireflection film between
that of the transparent resin used for device packaging and that of
the top layer of the semiconductor multilayer structure, the effect
of increasing the light output efficiency can be enhanced
further.
Here, in the conventional device, the semiconductor multilayer
structure has a refractive index of about 3.5 and the transparent
resin for plastic encapsulation has a refractive index of about 1.5
and hence there is a large difference in refractive index between
them. In this case, the critical angle associated with total
reflection of light traveling from the semiconductor multilayer
structure to the transparent resin is small. In this embodiment,
the critical angle associated with total reflection can be made
large by interposing between the semiconductor multilayer structure
and the transparent resin an antireflection film whose refractive
index (1.5 to 3.5) is intermediate between their refractive
indices. Thereby, the light output efficiency can be increased.
Moreover, the light output efficiency can be further increased by
making the antireflection film surface rough.
The emitted wavelength of the LED is not restricted to the
wavelength of green light. The above effects were also confirmed by
products adapted for visible light other than green light.
Concerning the shape of protrusions and recesses (irregularities)
of the antireflection film which are of the size of less than the
emitted wavelength, we confirmed that any of surface configurations
shown in FIGS. 14A to 14E allowed the light output efficiency to be
increased.
Besides InGaAlP, an InGaAlAs-based material, an AlGaAs-based
material or a GaP-based material may be used as the LED material.
Further, to prepare the antireflection film, TiO.sub.2, TaO.sub.2
or ZrO.sub.2 may be added to acrylic resin.
[Sixth Embodiment]
FIG. 15 is a sectional view of a green LED according to a sixth
embodiment of the present invention.
In this embodiment, the conductivity type of each semiconductor
layer is made opposite to that of the corresponding semiconductor
layer in the fifth embodiment. The method of manufacture is
substantially the same as in the fifth embodiment. That is, onto a
p-type GaAs substrate 120 of 250 .mu.m in thickness, a p-type GaAs
buffer layer 121 of 0.5 .mu.m in thickness, a p-type In.sub.0.5
Al.sub.0.5 P cladding layer 122 of 0.6 .mu.m in thickness, an
undoped In.sub.0.5 (Ga.sub.0.55 Al.sub.0.45).sub.0.5 P active layer
123 of 1.0 .mu.m in thickness, an n-type In.sub.0.5 Al.sub.0.5 P
cladding layer 124 of 1.0 .mu.m in thickness, an n-type InGaP
current diffusing layer 125 of 1.0 .mu.m in thickness and an n-type
GaAs contact layer 126 of 0.1 .mu.m in thickness are grown in the
same chamber.
As in the fifth embodiment, an antireflection film 127 having a
refractive index of 2.0 is formed on the contact layer 126 by spin
coating and then subjected to press shaping using a metal mold so
that its surface becomes roughened. A portion of the antireflection
film 127 is removed in the place where an electrode is to be formed
and an upper electrode (n-side electrode) 128 is formed on the
exposed portion of the contact layer 126. The GaAs substrate 120 is
formed underneath with a lower electrode (p-side electrode) 129.
The resulting wafer is scribed to obtain chips. Each chip is
encapsulated with resin material after having been subjected to a
wire bonding step.
Even with such a structure, the light output efficiency was
increased by a factor of about 2.5 as in the sixth embodiment. The
same effects were also confirmed by products adapted for visible
light other than green light. Further, we confirmed that any of the
surface configurations shown in FIGS. 14A to 14E allowed the light
output efficiency to be increased.
[Seventh Embodiment]
In FIG. 8, there is illustrated, in sectional view, the structure
of a surface-emitting LD according to a seventh embodiment of the
present invention.
First, on an n-type GaAs substrate 130 of 250 .mu.m in thickness
are sequentially grown an n-type GaAs buffer layer 131 of 0.5 .mu.m
in thickness and a multilayer reflecting layer 132 consisting of
stacked n-In.sub.0.5 Al.sub.0.5 P/n-GaAs films. Subsequently, an
n-type In.sub.0.5 Al.sub.0.5 P cladding layer 133 of 0.6 .mu.m in
thickness, an undoped MQW active layer 134 of In.sub.0.5
(Ga.sub.0.55 Al.sub.0.45).sub.0.5 P/In.sub.0.5 Ga.sub.0.5 P, and a
p-type In.sub.0.5 Al.sub.0.5 P cladding layer 135 of 0.6 .mu.m in
thickness are grown in sequence. Subsequently, a multilayer
reflecting layer 136 consisting of stacked p-In.sub.0.5 Al.sub.0.5
P/p-GaAs films, a p-type In.sub.0.5 Al.sub.0.5 P current diffusing
layer 137 of 1.0 .mu.m in thickness and a p-type GaAs contact layer
138 of 0.1 .mu.m in thickness are grown in sequence. The epitaxial
layers from the buffer layer 131 to the contact layer 138 are grown
in the same chamber.
Next, a resist mask in the form of a stripe is formed on the
contact layer 138. After that, the layers through the n-type
cladding layer 133 are then etched away using the resist mask to
thereby form a laser ridge. Subsequently, an SiO.sub.2 insulating
film 141 of 0.5 .mu.m is formed except the top of the ridge and
then a p-type electrode (Zn-containing Au) is evaporated. Using a
resist mask, an upper electrode 142 is then formed. The upper
electrode 142 comes into contact with the peripheral portion of the
contact layer 138 and the central portion of the contact layer is
exposed.
Next, an antireflection film 144 having a refractive index of 2.0
and prepared by adding TiO.sub.2 to polyimide resin is formed on
the contact layer 116 by spin coating and its surface is then press
shaped by a metal mold having protrusions which are less in size
than the wavelength of emitted light. Thereby, the surface
roughness (PV value (max-min)) of the antireflection film 117 is
made smaller than the wavelength of emitted light. Afterward, the
unnecessary portion of the antireflection film 144 is removed.
Next, the GaAs substrate 130 has its rear side polished to a
thickness of 100 .mu.m and then formed underneath with an n-side
electrode (Ge-containing Au) 143. After that, the resulting
structure is subjected to a heat treatment at 450.degree. C. for 15
minutes in an Ar atmosphere. Subsequently, the resulting wafer is
scribed to obtain chips. Each individual chip is assembled and
housed in a package made of epoxy-based resin (n is about 1.5).
In this embodiment, as in the fifth embodiment, the light output
efficiency can be increased significantly by forming the
antireflection film 144 which is intermediate in refractive index
between the underlying semiconductor layers and the sealing resin
and has its surface made rough. Concerning the surface topology of
the antireflection film, we confirmed that any of surface
configurations shown in FIGS. 14A to 14E allowed the light output
efficiency to be increased as in the fifth embodiment.
Besides InGaAlP, an InGaAlAs-based material, an AlGaAs-based
material or a GaP-based material may be used as the semiconductor
material. Further, to prepare the antireflection film, TiO.sub.2,
TaO.sub.2 or ZrO.sub.2 may be added to acrylic resin.
[Modifications]
The present invention is not restricted to the embodiments
described so far. Although, in the first and fourth embodiments,
the PH.sub.3 partial pressure is set at 10 Pa to make the crystal
surface rough, it may lie in the range of 1 to 20 Pa. In the third
embodiment, to make the crystal surface rough, annealing is
performed with AsH.sub.3 introduced. The gas used on annealing is
not restricted to AsH.sub.3. Any other gas may be used provided
that it contains hydrogen and a group V element different from a
group V element that constitutes the semiconductor layer whose
surface is to be roughened. The method of making the crystal
surface rough is not restricted to reducing the PH.sub.3 partial
pressure and annealing after crystal growth. It is also possible to
process randomly the surface of the semiconductor layer with a
grinder having a point angle of less than 120 degrees.
The protrusions need not be restricted to circular cones but may be
pyramidal ones, such as triangular pyramids, square pyramids,
hexagonal pyramids, etc. The protrusions need not necessarily be
formed over the entire surface on the light output side. However,
it is desired that the percentage of the area occupied by the
protrusions on the light output surface be as large as possible. If
the percentage is 50% or more, a satisfactory result will be
obtained.
The light output efficiency is proportional to the occupied area;
thus, if the occupied area by protrusions is 50% or less, the light
output efficiency will be halved (1.1 times or less). If the pitch
of the protrusions is in a range of 0.2 to 0.5 .mu.m, the light
output efficiency is reduced (1.1 times or less). if the pitch is
less than 0.2 .mu.m, the graded index effect will occur.
In the fifth, sixth and eighth embodiments, a metal mold having
protrusions and recesses is used to make the surface of the
antireflection film rough; instead, it is also possible to roughen
the surface of an antireflection film already formed with a
grinder. In this case, various materials other than resin can be
used for the antireflection film.
The requirement that the surface roughness (PV value) be ranged
from 200 nm to emitted wavelength is not necessarily applied to the
antireflection film alone. The requirement may be applied to any
other layer on the light output side of the semiconductor
multilayer structure. Specifically, the requirement may be applied
to the current diffusing layer or the contact layer. That is, in
the first through fourth embodiments, the surface roughness (PV
value) of the roughened surface may be set to emitted wavelength or
less. Further, the requirement that the surface roughness (PV
value) be the emitted wavelength or less and the requirement that a
be 30 degrees or more may both be satisfied.
If current can be diffused sufficiently to regions other than just
below the upper electrode between the upper electrode and the
active layer, the current diffusing layer is not necessarily
required; it may be omitted. The materials, compositions and
thickness of semiconductor layers forming a light emitting device
may be changed according to specifications.
Although the embodiments have been described taking transparent
resin-based encapsulation by way of example, this is not
restrictive. In the case of no resin-based encapsulation, it is air
that comes directly into contact with the antireflection film. In
this case as well, since there is a large difference in refractive
index between the semiconductor multilayer structure and air, the
effect of increasing the light output efficiency resulting from the
formation of the antireflection film could be expected
likewise.
Additional advantages and modifications will readily occur to those
skilled in the art. Therefore, the invention in its broader aspects
is not limited to the specific details and representative
embodiments shown and described herein. Accordingly, various
modifications may be made without departing from the spirit or
scope of the general inventive concept as defined by the appended
claims and their equivalents.
* * * * *